In recent years, the lower fuel consumption requirement for automobiles became stricter from the standpoints of energy saving, environmental protection, etc. In this connection, a rubber material for tires, of small rolling friction and superior abrasion resistance is required strongly.
It is known that in diene rubbers such as polybutadiene (BR), styrene-butadiene copolymer rubber (SBR) and the like, their abrasion resistance and resilience can be improved by making larger the molecular weight. Diene rubbers of larger molecular weight are improved in these properties; however, they have drawbacks such as lower processability when unvulcanized, increased hardness at low temperatures and the like, making difficult their practical use in many cases. Hence, there were proposed methods of adding, as a processability improver, a liquid BR, a liquid SBR or the like all having a molecular weight of 50,000 or less (e.g. JP-A-4-277537, JP-A-6-65418 and JP-A-6-65419). With these methods, however, addition of a large amount of a liquid rubber reduces the superior abrasion resistance and resilience inherently possessed by high-molecular polymers, and the other drawback of high-molecular polymers, i.e. the low-temperature hardness is not improved noticeably.
Meanwhile, JP-A-61-271338 disclosed a rubber composition comprising a branched polymer and a modified polymer, obtained by subjecting a diene polymer containing active lithium in the molecule, obtained by anionic polymerization using n-butyllithium, to tin coupling of required level and then modifying the remaining active lithium with a particular cyclic urea compound. In this rubber composition, however, the branched polymer has a low weight-average molecular weight not higher than 600,000; therefore, the rubber composition is insufficient in resiliency, abrasion resistance, etc. and, moreover, is not fully satisfactory in processability, low-temperature hardness, etc.